Balloons mounted on the distal ends of catheters are widely used in medical treatment. The balloon may be used widen a vessel into which the catheter is inserted or to force open a blocked vessel. The requirements for strength and size of the balloons vary widely depending on the balloon's intended use and the vessel size into which the catheter is inserted. Perhaps the most demanding applications for such balloons are in balloon angioplasty in which catheters are inserted for long distances into extremely small vessels and used to open stenoses of blood vessels by balloon inflation. These applications require extremely thin walled high strength relatively inelastic balloons of predictable inflation properties. Thin walls are necessary because the balloon's wall and waist thicknesses limit the minimum diameter of the distal end of the catheter and therefore determine the limits on vessel size treatable by the method and the ease of passage of the catheter through the vascular system. High strength is necessary because the balloon is used to push open a stenosis and so the thin wall must not burst under the high internal pressures necessary to accomplish this task. The balloon must have some elasticity so that the inflated diameter can be controlled, so as to allow the surgeon to vary the balloon's diameter as required to treat individual lesions, but that elasticity must be relatively low so that the diameter is easily controllable. Small variations in pressure must not cause wide variation in diameter.
While angioplasty balloons are considered inelastic relative to balloons used in most other applications, there is in the art a general classification of such balloons based on their expandability or "compliance" relative to each other. As used herein, "non-compliant" balloons are the least elastic, increasing in diameter about 2-7%, typically about 5%, as the balloon is pressurized from a inflation pressure of about 6 atm to a pressure of about 12 atm, that is, they have a "distension" over that pressure range of about 5%. "Semi-compliant" balloons have somewhat greater distensions, generally 7-16% and typically 10-12% over the same pressurization range. "Compliant" balloons are still more distensible, having distensions generally in the range of 16-40% and typically about 21% over the same pressure range. Maximum distensions, i.e. distension from nominal diameter to burst, of various balloon materials may be significantly higher than the distension percentages discussed above because wall strengths, and thus burst pressures, vary widely between balloon materials. The 6-12 atm inflation range is used in the present application to allow direct comparison of the compliance attributes of various balloons.
The strength of the polymer materials used in the balloons varies widely. The strongest balloons are also the most inelastic, being made of highly orientable polymers such as polypropylene, polyethylene terephthalate or other phthalate polyesters or copolyesters, and nylons. Tensile wall strengths are commonly 20,000-50,000 psi.
Commercial angioplasty balloons made of such materials with nominal diameters in the range of 1.5-4.5 mm have distensions in the non-compliant to semi-compliant range and can often be rated to pressures of 16 atm or higher without risk of bursting (actual burst pressures may exceed 20 atm). Generally, however, as compliance increases the wall strength decreases. Other semi-compliant and compliant balloons are made of less highly orientable polymers such as ethylene-vinyl acetate, polyvinyl chloride, olefin copolymers and ionomer resins. The wall strengths of balloons made from these less orientable materials are still lower than those made from the highly orientable polymers, commonly in the range of 6,000-15,000 psi, resulting in lower rated maximum inflation pressures of 9-10 atm.
The particular distension and maximum pressure attributes of a balloon are also influenced both by polymer type and by the conditions under which the balloon is blown. Angioplasty balloons are conventionally made by blowing a tube of polymer material at a temperature above its glass transition temperature. For any given balloon material, there will be a range of distensions achievable depending on the conditions chosen for the blowing of the balloon.
In U.S. Pat. No. 4,906,244 to Pinchuck there are described balloons of nylon (i.e. aliphatic polyamide) materials, such as nylon 12, nylon 11, nylon 9, nylon 6/9 and nylon 6/6. Like all other polymer materials the distensions of these balloons can be determined, within a range, by controlling blowing conditions such as initial dimensions of tubing, prestretching, hoop ratio and heat set conditions. The data in the reference show that compliance characteristics can be obtained ranging from non-compliant to semi-compliant characteristics and that wall strengths of greater than 15,000 can be obtained. The reference suggests that higher compliances can be achieved with nylon materials but there is no indication of what other nylons or other balloon forming conditions could be employed to do so.
It has also been suggested to prepare balloons of thermoplastic elastomers in U.S. Pat. No. 4,254,774 to Boretos, and polyamide elastomers have been mentioned among a number of possible balloon materials suggested in U.S. Pat. No. 5,250,069 to Nobuyoshi, et al, but there are many of such thermoplastic elastomer polymers and before the invention hereof it has been expected that performance of balloons made from these materials would not be generally any better than high to intermediate compliance made from conventional thermoplastic polymers such as polyethylene ionomer, polyvinyl chloride, polyethylene or ethylene-vinyl acetate.
In U.S. Pat. No. 5,290,306 polyester ethers and polyetheresteramide polymers of Shore D hardness less than 55 have been proposed for use as a sleeve or co-extruded outer layer to a balloon of a biaxially oriented nylon or polyethylene terephthalate (PET) material, so as to provide the balloon with improved softness and pin-hole and abrasion resistance.
Polyurethane block copolymers having flexural modulus of about 190,000 and an ultimate elongation of 250% are disclosed as balloon materials in EP 0592885 and mention is made of also using polyester block copolymers or polyamide block copolymers but no suggestion is made that such alternative copolymers could be usefully employed if their flexural modulus was substantially lower or their ultimate elongation was substantially higher than the disclosed polyurethane block copolymers.